From U.S. Pat. No. 2,179,181 it is known that tertiary amine-oxides have the property of dissolving cellulose and that cellulose mouldings such as fibres can be obtained from these solutions by precipitation. A process for the preparation of such solutions is known for example from EP-A 0 356 419. According to this publication a suspension of cellulose is firstly prepared in an aqueous tertiary amine-oxide. The amine-oxide contains up to 40 wt % water. The aqueous cellulose suspension is heated and water is removed under reduced pressure until the cellulose dissolves.
When preparing cellulose fibres, it is known from DE-A 2 844 163 that an air gap can be provided between the spinning die and the precipitation bath to achieve drawing at the die. This drawing at the die is necessary because drawing of the fibres becomes very difficult after contact of the shaped spinning solution with the aqueous precipitation bath. The fibre structure which is set in the air gap is fixed in the precipitation bath.
A process for the preparation of cellulose fibres is also known from DE-A 2 830 685, wherein a solution of cellulose in a tertiary amine-oxide is shaped in hot condition to give filaments which are cooled with air and then introduced into a precipitation bath in order to precipitate the dissolved cellulose. The surface of the spun fibres is also wetted with water to reduce their tendency to adhere to neighbouring fibres.
A device and a process of the type mentioned above for the preparation of seamless tubular film is known from WO 93/13670. According to this known process the cellulose solution is passed through an extrusion die having a ring-shaped extrusion orifice to give a tube which is drawn over a cylindrical mandrel and then introduced into a precipitation bath. To ensure that the extruded tube does not adhere to the surface of the mandrel, its surface is covered with a film of water so that the inner side of the tube coagulates and slides over the cylindrical mandrel. This has the disadvantage however that the water used to wet the surface of the mandrel can rise to the extrusion orifice and wet the die lip, which not only leads to unwanted coagulation in the actual extrusion process but also leads to the extrusion die being cooled. This is undesirable because the cooled die cools the solution to be extruded thus increasing its viscosity, so that trouble-free extrusion to give sheet with a uniform thickness is no longer possible. In addition the device previously cited must undergo a costly rebuild when for instance sheets of varying thicknesses are to be prepared.
A further disadvantage of the device described in WO 93/13670 is that it does not allow high production rates. This is because the cooling of the extrudate is not efficient enough; the reason is that the heat cannot be dissipated quickly enough from the hot extruded sheet.
On account of their extraordinarily high viscosity among other reasons, the cellulose solutions which are to be extruded must be heated to temperatures above 110.degree. C. so that they may be processed at all. After the extrudate emerges from the die lips the extruded solution should cool down to a certain extent and solidify so that it can be drawn before its introduction into the precipitation bath, ie, before coagulation. If the cooling is not efficient then the residence time of the extruded solution in the air gap must be correspondingly lengthened which can be achieved only by reducing the draw-off speed. If on the other hand extrusion is carried out at a lower temperature then problems arise with the distribution of material in the extrusion device.